1. Field of the Invention
The present invention relates to a metal binder effective in the injection molding of products using a metal powder as an ingredient and to a molding composition in which this metal binder is blended.
2. Description of the Related Art
In conventional powder metallurgy wherein powders of various metals (the word "metal" used here also includes an alloy and a sintered hard alloy) are molded and then sintered to produce sintered metal articles, the molding is generally carried out by press molding. However, press molding is not satisfactory for forming complicatedly-shaped objects. Besides, press molding cannot assure a high sintering density and sufficient strength in the succeeding sintering step.
To solve these problems, injection molding has been proposed in the field of metal powder molding and some proposals have actually been put into practice. Injection molding techniques for metal powders are disclosed, for example, in U.S. Pat. Nos. 4,305,756, 4,404,166, 4,415,528, 4,445,936, 4,602,953, and 4,661,315. The injection molding technique can form complicated shapes and can be utilized for the molding of various objects. In addition, this technique has the advantages that the kneading, feeding, and molding of the metal powder and binder are carried out in one process, a high molding accuracy can be attained, and the forming step can be omitted or simplified.
In this connection, it is to be noted that the binders used in the conventional powder metallurgy art include polymeric materials such as ethylene-vinyl acetate copolymer, poly(meta)acrylate, polypropylene, plasticizers such as dibutyl phthalate, and waxes such as paraffin wax. Such binders are used also in the injection molding of the metal powders.
These binders heretofore used, however, have the disadvantages that they are rather difficult to be removed, it takes considerable time for them to be removed, cracking or swelling are liable to occur when debindering, and a high sintering density and high dimensional accuracy are not assured.
Shaped articles formed by injection molding using a conventional binder have a further disadvantage that they cannot retain their shapes due to the fluidity caused by softening of the articles unless debindering and sintering are carried out with the articles placed in the powders.
For these reasons, it is difficult to use injection molding in the field of powder metallurgy, though injection molding is known to be desirable. It is especially difficult, almost impossible, to utilize injection molding for the production of precision sintered articles.
On the other hand, ceramic binders using adamantane and/or trimethylene norbornone as an auxiliary binder agent is known in the field of the preparation of ceramic products (Japanese Patent Application Publication (Kokai) No. 62-3064).